Can’t upgrade to 2.8.1 using Automatic Upgrade: Error in Maintenance Mode

I’m trying to upgrade to 2.8.1 on a client’s site and I’m using the Automatic Upgrade plugin and everytime I get to the point where I enter Maintenance Mode and all the plugins are supposed to be turned off, I get an error and it doesn’t complete the process. Here’s what the logs say:
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Task Name: maintmode
Task Description: Puts the site into maintenance mode
Task Status: Failed
Task Start Date: 2009-07-11 15:46:42
Task End Date: 2009-07-11 15:46:42
Task Log:
Putting the site into maintenance mode /www/sites/www.yourwebsite.net/files/html/wp-content/plugins/wordpress-automatic-upgrade
Copying /www/sites/www.yourwebsite.net/files/html/index.php to /www/sites/www.yourwebsite.net/files/html/index.php.wpau.bak
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With yourwebsite.net representing the actual name of the site.

All the other processes are fine at that point. I tried changing the folder permissions to 777 but that didn’t work. So what will?